feat: add xAI handling and response processing

relay/channel/xai/dto.go

@@ -0,0 +1,14 @@
+package xai
+
+import "one-api/dto"
+
+// ChatCompletionResponse represents the response from XAI chat completion API
+type ChatCompletionResponse struct {
+	Id                string `json:"id"`
+	Object            string `json:"object"`
+	Created           int64  `json:"created"`
+	Model             string `json:"model"`
+	Choices           []dto.ChatCompletionsStreamResponseChoice
+	Usage             *dto.Usage `json:"usage"`
+	SystemFingerprint string     `json:"system_fingerprint"`
+}